Astronomers in the nineteenth century noticed that Mercury's observed orbital motion differed slightly from what they had calculated. Many thought that these anomalies could be explained by an unseen planet (they called it Vulcan), which would be closer to the Sun than Mercury. The gravitational interactions between Mercury and this hypothetical planet could explain Mercury's observed behavior. A number of astronomers searched for Vulcan, but it was never found. Mercury's orbital motion was eventually explained by Albert Einstein, with general relativity.

There are some smaller objects (asteroids) that go near Mercury. As of July 2015, there are over 200 known asteroids that come within 46 million kilometers of the Sun, meaning that they cross Mercury's orbit, and they can get closer to the Sun than Mercury ever does. (Over the course of Mercury's orbit, its distance from the Sun varies between 46 million km and 70 million km; that's 0.31 to 0.47 AU.)

Wait - doesn't mean that Mercury has not "cleared its orbit"? According to the IAU definition, shouldn't Mercury be considered a dwarf planet?

In order for Mercury to be "demoted" from planet to dwarf planet, there would have to be another object of comparable mass that crosses Mercury's orbit (or comes very close to doing so). The asteroids that pass near Mercury are much less massive than Mercury.

But isn't it possible that there is another planet-sized object in there? How can astronomers be sure that "Vulcan" doesn't exist?

It is difficult to observe objects that are close to the Sun, but we have enough observations to say that there cannot be any other bodies in the inner solar system that are large enough to be considered planets.

Asteroids that orbit completely interior to Mercury would be called vulcanoids. As of July 2015, no vulcanoids have ever been found. A study in 2013, using data from NASA's Sun-observing STEREO spacecraft, concluded that there can be no vulcanoids larger than six kilometers in diameter, since none were ever seen by STEREO. The closest known thing to a vulcanoid would be asteroid 2007 EB26, which occasionally gets closer to the Sun than Mercury, but it still spends most of its time outside Mercury's orbit.

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